Conclusion: Our data showed that measurement of fetal soft tissue thickness and subcutaneous fat tissue thickness of the upper arm as well as the ratio of subcutaneous fat tissue thickness to humerus length were good predictors of fetal macrosomia in diabetic pregnancies.

Gestational diabetes mellitus is defined as glucose intolerance diagnosed first during pregnancy using a 75g glucose tolerance test according to the International Association of the Diabetes and Pregnancy Study Group`s criteria [1]. Pre-gestational diabetes mellitus (PGDM) and gestational diabetes mellitus (GDM) are among the most common metabolic diseases during pregnancy. They are associated with an increased risk of fetal macrosomia and perinatal complications for the fetus and mother such as shoulder dystocia, prolonged labour, instrumental delivery, high-grade perineal tears and caesarean section [2,3]. Furthermore, macrosomic and large-for-gestational age (LGA) fetuses have an increased risk of developing a metabolic syndrome (obesity, dyslipidemia, hypertension, glucose intolerance) during childhood and adolescence [4,5]. Poor maternal glycemic control is correlated with fetal hyperinsulinemia leading to an increase in subcutaneous fat, which in turn is responsible for fetal macrosomia [6,7].

In order to be able to predict perinatal complications during childbirth several studies have addressed the diagnosis of fetal macrosomia using different ultrasound based methods [8]. Fetal weight and abdominal circumference, the latter supposedly the most sensitive individual parameter, are commonly used in the prediction of excessive fetal growth. Both estimates, however, show a wide range of variation and may differ from the actual fetal weight by 10-15% [3,9].

In neonates, skinfold measurements are commonly used to assess perinatal body composition and body fat mass [10,11]. Therefore, ultrasound measurement of fetal subcutaneous fat tissue thickness would seem to be a useful method to predict growth abnormalities such as fetal macrosomia in diabetic pregnancies, in which excessive growth is mainly associated with increased fetal subcutaneous fat tissue depots [9]. Measurements of subcutaneous fat tissue, femur and humerus length have the advantage that they are simple to perform and their accuracy generally is not affected by labour or rupture of membranes in contrast to fetal weight estimation which is based on the accuracy of measurements of femur, head and abdominal circumference.

Rotmensch et al [12] examined the screening efficacy of the ratio of subcutaneous fat tissue thickness of the thigh to femur length in non-diabetic pregnancies. They showed that the ratio was a poor predictor of fetal macrosomia in the non-diabetic pregnancy but they did hypothesize that fetal subcutaneous tissue thickness would show a better correlation to birth weight in diabetic mothers.

In this study we measured fetal abdominal circumference, anterior abdominal wall thickness, soft tissue thickness and subcutaneous fat tissue thickness of the upper arm (major tubercle level) and thigh (greater trochanter level), and the ratios between subcutaneous fat tissue thickness to the length of humerus and femur in diabetic pregnancies between 25 and 39 weeks of gestation to evaluate their usefulness in predicting fetal macrosomia.

Materials and Methods

One hundred and fifty-eight patients received one ultrasound examination, performed by the same investigator between February 2014 and May 2016. Inclusion criteria were gestational age between 25 and 39 weeks and a singleton pregnancy complicated by diet-controlled GDM, insulin-dependent GDM, PGDM Type 1 or PGDM Type 2. Exclusion criteria were fetal malformations, drug dependence, labour or if patients were incapable of giving consent. Twenty patients were excluded retrospectively from the final analysis due to fetal growth retardation, determined using Hadlock`s formula (Log₁₀ (weight) = 1.326 – 0.00326 AC × FL + 0.0107 HC + 0.0438 AC + 0.158 FL) [13,14] or incomplete perinatal data (moving out of the area). All patients provided written informed consent and the study was approved by our hospital`s ethics committee. Gestational age was calculated from the last menstrual period, confirmed by first trimester crown-rump length measurement and corrected when required [15]. All women had an uneventful pregnancy.

A glucose tolerance test using a 75g glucose load was performed between gestational weeks 24-28 according to the International Association of the Diabetes and Pregnancy Study Group`s criteria [1]. Diagnosis of GDM was based on at least one abnormal glucose measurement in the glucose tolerance test (fasting glucose ≥ 92 mg/dl, plasma glucose ≥ 180 mg/dl 1hour, ≥ 153 mg/dl 2 hours after glucose administration). Sixty-nine patients required insulin to treat their GDM, whereas 39 were able to achieve normal glucose levels with dietary counselling. Twenty patients had PGDM Type 1 and 10 patients had PGDM Type 2. BMI values ranged from 15.24 to 49.5. Each patient was examined by transabdominal ultrasound examination using an Aplio 500 ultrasound device (Toshiba Medical Systems Europe). Broad band curved array transducers (3.5 MHz and 6MHz) were used to perform routine fetal biometry measurements and measurements of subcutaneous tissue thickness. Hadlock`s formula was used to estimate fetal weight [13,14]. Abdominal circumference (AC) was measured in a cross sectional view of the upper abdomen visualising the stomach, one pair of ribs and the vertebrae as well as the umbilical vein at the level of the portal sinus. Measurements of anterior abdominal wall (AAW) thickness were made 2 cm from the umbilical vein from the skin surface to the anterior aspect of the liver (Fig 1). Adequate magnification was defined as the abdominal circumference filling the entire screen area. Soft tissue and subcutaneous fat tissue of the upper arm (HumST, HumSC) at the major tubercle level and thigh (FemST, FemSC) at greater trochanter level were measured in longitudinal views between skin surface and lean muscle below (Fig 2). Actual birth weights above the 90th percentile for gestational age were considered macrosomic [16].

Statistical analysis

AGA and macrosomic fetuses were compared using the non-parametric Wilcoxon-Mann-Whitney-U test and the Fisher-Freeman-Halton test as shown in Table 1 (demographic data) and Table 2 (sonographic measurements).

Receiver operating characteristic (ROC) curves were constructed by plotting the sensitivity of the listed parameters against their specificity. The curves illustrate sensitivity and specificity of the parameters for the detection of macrosomia at cut-off points defined in Table 3. The areas under the curve (AUC) were calculated and compared as described by DeLong et al. [17].

Spearman`s rank correlation coefficient was used to assess the correlation of gestational age and birth weight percentiles with the fetal biometric measurements: abdominal circumference (AC), anterior abdominal wall (AAW) thickness, soft tissue thickness of the upper arm and thigh (HumST, FemST), subcutaneous fat tissue thickness of the upper arm and thigh (HumSC, FemSC), and the ratios of subcutaneous fat tissue thickness to humerus and femur length (HumSC/HL and FemSC/FL).

To adjust for correlations with gestational age, we utilized residuals obtained from linear regression models, where gestational age was used as an independent variable and measurement of AC, AAW thickness, HumST, FemST, HumSC, and FemSC thickness, and the ratios HumSC/HL, and FemSC/FL were the corresponding dependent variables. Residuals were checked for normality using the Kolmogorov-Smirnoff-Lilliefors-test.

All tests were two-sided and a p-value of ≤ 0.05 was considered statistically significant.

Of the 138 patients examined mean maternal age at the time of investigation was 34.9 years (range 25-44 years), mean gestational age at examination was 32.8 weeks (range 25 – 38 weeks), mean gestational age of the new-borns was 38.5 gestational weeks (range 33 – 41 weeks) and mean birth weight was 3416 g (range 1840 g – 4810 g). At the time of the examination 39 GDM patients were able to achieve normal glucose levels with dietary counselling, 69 GDM patients were being treated with insulin, 20 patients had PGDM Type 1 and 10 had PGDM Type 2.

Table 1 shows demographic data for pregnancies resulting in macrosomic and appropriate-for-gestational age (AGA) new-borns. A higher percentage of macrosomic fetuses (35%) was found in patients with diabetes mellitus Type 1 (7 out of 20 fetuses), than in patients with GDM. A similar tendency albeit based on small patient numbers was found for diabetes mellitus Type 2 (20%, 2 out of 10 fetuses). Macrosomic newborns were younger at birth than AGA new-borns (37.4 vs 38.7 weeks, p= 0.003). Pregnancies resulting in macrosomic and normal-weight new-borns did not differ with regard to maternal age, gravidity, parity, BMI, gestational age at examination or sex of the new-borns.

The ROC curves for AC, AAW thickness, FemST, FemSC, the ratio FemSC/FL, HumST, HumSC and the ratio HumSC/HL are presented in Figures 3, 4 and 5, respectively. ROC curves were constructed by plotting sensitivity against specificity. The areas under the ROC curve, their respective p-values and the calculated cut-off points for fetal biometric measurements to predict fetal macrosomia, are listed in Table 3. The best possible prediction efficacy would be displayed by an area under the curve of 1.0 reflecting 100% sensitivity and 100% specificity.

To adjust for correlations with gestational age, we utilized residuals obtained from linear regression models, with gestational age as an independent variable and the fetal biometric measurements as corresponding dependent variables. Normality could be assumed in AC, AAW thickness, HumST, FemST, FemSC, and the ratio of FemSC/FL. In HumST and FemST normality of the residuals was obtained after log-transformation, and in HumSC after log-log-transformation. However, no normality was demonstrated by this method for the ratio of HumSC/HL. Hence, the residuals could be interpreted as age-adjusted percentiles. The ROC-curves that were constructed using age-adjusted residuals of AC, HumST and FemSC showed larger areas under the curve (AC: AUC=0.838, SD=0.057, 95%-confidence interval (0.726-0.949), p=0.137; HumST: AUC=0.736, SD=0.088, 95%-confidence interval (0.563-0.909), p=0.87; FemSC: AUC=0.765, SD=0.071, 95%-confidence interval (0.626-0.904), p=0.539) than when using the non-adjusted residuals, but the difference was not significant.

Discussion

Our study compared the usefulness of several published parameters in predicting fetal macrosomia in diabetic pregnancies and found that fetal soft tissue thickness and subcutaneous fat tissue thickness of the upper arm (HumST and HumSC) as well as the ratio of subcutaneous fat tissue thickness to humerus length (HumSC/HL) showed significant differences between macrosomic and AGA fetuses. Calculation of the area under the ROC curve confirmed these three parameters as the strongest predictors of fetal macrosomia when compared to abdominal circumference (AC), anterior abdominal wall (AAW) thickness, soft tissue thickness of the thigh (FemST), subcutaneous fat tissue thickness of the thigh (FemSC), and the ratio of subcutaneous fat tissue thickness to femur length (FemSC/FL) in diabetic pregnant women.

Sod et al. [18] and Landon et al. [19] reported that macrosomic neonates born to diabetic mothers show disproportionately larger shoulders, trunk and upper arms. This interesting observation agrees with our finding that macrosomic fetuses have longer humeri than AGA fetuses whereas the length of femora in both groups were similar. In the consideration which fetus might encounter perinatal complications, subcutaneous fat tissue thickness relative to the length of long bones is indeed an important parameter [12]: a short fetus with excessive fat deposition as frequently found in diabetes may be more prone to the complications than a long fetus of the same weight. Therefore, we tested the possibility of enhancing the sensitivity of sonographic soft tissue and subcutaneous fat tissue measurements by calculating the ratio of subcutaneous fat tissue to length of humerus and femur and found this parameter to be more sensitive than subcutaneous fat or bone length alone. Similar calculations were made by Santaloya-Forgas et al. [20] in normal pregnancies. They reported that the ratio of Fem SC/FL was an improvement in the detection efficacy for LGA fetuses over AGA fetuses with respect to AC and estimated fetal weight (EFW). In agreement with this group we find that the ratio of subcutaneous fat to the respective long bone improves the recognition of macrosomic fetuses although we find the ratio of HumSC/HL superior to the ratio of FemSC/FL reported by Santaloya-Forgas et al [20].

Chauhan et al [8] examined HumST and the ratio of FemSC/FL and found them to be inferior to sonographically estimated fatal weight or clinical predictions in identifying macrosomic fetuses. The difference between their observations and ours may be explained by different measuring sites (Table 4) or different population characteristics (diabetic vs non-diabetic pregnancies), or their definition of macrosomic fetuses at an expected birth weight of >4000g, whereas we defined macrosomia as being above the 90th percentile. As shown in Voigt`s growth chart [16] birth weights below 4000g can still be above the 90th percentile for age at birth in preterm delivery. Also, the effective treatment of diabetes during pregnancy may have led to reduced prevalence of macrosomia at birth in our study.

Our data show that fetal subcutaneous fat tissue thickness of the upper arm and the ratio of subcutaneous fat tissue thickness to humerus length are superior predictors of fetal macrosomia in diabetic pregnancies when compared to abdominal circumference, anterior abdominal wall thickness, soft tissue thickness and subcutaneous tissue thickness of the femur and the ratio of subcutaneous tissue thickness to femur length. Currently, established formulas for estimation of fetal weight do not include the disproportionately greater amount of subcutaneous fat tissue, which is so important in fetuses of diabetic mothers [18,19]. The consideration of subcutaneous fat may improve the prediction of fetal macrosomia in diabetic pregnancies.